Planar waveguide devices are important components of optical fiber communication systems. Such systems typically comprise long lengths of fiber for transmission and often use planar waveguide devices to perform a variety of processes such as filtering, multiplexing signal channels, demultiplexing, compensating chromatic dispersion and compensating polarization dispersion.
An optical fiber is typically in the form of a thin strand of glass having a central core of circular cross section peripherally surrounded by concentric cladding glass. The core has a higher refractive index than the cladding so that the light is retained in the core by total internal reflection and propagates in a fiber mode. For long distance transmission the core dimensions are typically chosen so that the light propagates in a single circular mode.
A planar waveguide device, in contrast, may be formed from a layer of silicon surrounded by a silicon dioxide cladding layer. The core is typically of rectangular cross section. The core region is formed, as by etching of a masked surface, into a patterned configuration that performs a desired function. In order to permit small radius curves, and thus compact functionality, the difference in refractive index of the planar waveguide core and the index of the cladding is typically substantially greater than the corresponding difference for optical fibers. The planar waveguide is said to be high delta where delta (A) is given by the core index less the cladding index, all divided by the core index.